Optically variable surface pattern

ABSTRACT

An optically variable surface pattern includes at least one graphic representation producing an achromatic impression when viewed in visible light over a certain angular range without noticeable color fringes occurring in the adjoining angular ranges. A plane surface portion includes a grating structure which disperses incident light with comparable intensity into a cone within a predetermined angle range regardless of differing wavelength. An overlap of several successive high orders of diffraction results in a recombination of the dispersed light to white light at any diffraction angle within the cone. The surface portion viewed from a direction within the cone reflects white light, in contrast to a simple flat mirror which has a very narrow range of specular reflection. At viewing angles outside the cone, the surface portion is dim or dark grey. The shape of the surface portion is then recognized as an area white lit or dark depending upon a particular viewing angle relative to incident light.

BACKGROUND OF THE INVENTION

The invention relates to an optically variable surface pattern of thekind set forth in the classifying portion of claim 1.

Such optically variable surface patterns with a microscopically finerelief structure are suitable for example for increasing the level ofsecurity against forgery and for conspicuously identifying articles ofall kinds and can be used in particular in relation to value-bearingpapers or bonds, identity cards, payment means and similar articles tobe safeguarded.

A surface pattern of the kind set forth in the classifying portion ofclaim 1 is known from EP 375 833. The surface pattern which is embossedin the form of a light-modifying relief structure into a carrier issubdivided into grid areas. Each grid area is divided into a number n ofsurface portions, wherein each surface portion is associated with apixel of one of n representations and wherein each has a respectivediffraction element which contains items of information about achromaticity, a brightness value and a viewing direction. The nrepresentations are composed of beams of diffracted light which becomevisible at n different viewing directions. In order that arepresentation becomes visible only at a single viewing direction thecorresponding relief structures are of an asymmetrical profile shape.

EP 360 969 discloses a diffraction element which has surface portionswith colours of high luminosity. The surface portions contain reliefstructures which are in the form of diffraction gratings with anasymmetrical profile shape, for example with a sawtooth-shaped profileconfiguration. The diffraction gratings reflect incident lightpredominantly in the first diffraction order. For that reason thediffraction gratings change their colour with a varying direction ofincidence of the light and a varying direction of view on the part of anobserver. The achievable degree of asymmetry, that is to say the ratioof the level of intensity of the light diffracted into the plus firstdiffraction order to the intensity of the light diffracted into theminus first diffraction order is typically 3:1 and at most 30:1.

DE 25 55 214 discloses optical markings which modify incident lightessentially not by diffraction but by reflection or optical refractionon the basis of the laws of geometrical optics. With line spacings of 10to 100 microns however those configurations already give profile heightsof several or several tens of micrometres, at moderate reflectionangles.

It is known from the technical literature, for example from the book"Diffraction Gratings", M. C. Hutley, Chapter 2, pages 13-56, ISBN0-12-362980-2 that light of a wavelength λ which is incident on agrating structure from a direction of incidence is diffracted inaccordance with the following relationship:

    sin(θ.sub.m)=sin(θ.sub.i)+m*λ/d         (1)

wherein d denotes the grating period, θ_(m) and θ_(i) denote theintermediate angles between the line normal to the surface with thegrating structure and the diffracted beam m and the incident beam irespectively and the integral index m denotes the diffraction order.There are only a finite number of diffraction orders. Accordinglypolychromatic light is resolved by the grating structure into itsspectral colours, that is to say light of different wavelengths isdiffracted into different directions. Now various methods are known fordiffracting the light of different wavelengths into the same directionin order within certain limits to avoid spectral colour resolution whichis perceptible by the eye and thereby to achieve an achromaticimpression. They are based on using grating structures with differentgrating periods. For example it is possible for grating structures withgrating periods d₁, d₂ and d₃ to be arranged in mutually juxtaposedrelationship in grid areas. The size of the grid areas is so selectedthat the grid areas are not separately perceptible by the human eye froma normal viewing distance of 30 cm. The periods d₁, d₂ and d₃ of thegratings are so selected that the spectra thereof are in superposedrelationship in a predetermined viewing direction, more specifically insuch a way that the diffraction directions of the red spectral componentof the grating structure 1, the green spectral component of the gratingstructure 2 and the blue spectral component of the grating structure 3are the same for a diffraction direction. The individual gratingstructures do not have to be arranged in mutually juxtaposedrelationship but they can also be in mutually superposed relationship asfor example in the case of holograms. Juxtaposition can also be replacedby a local, repetitive variation of the grating constant: the surfacewhich is to appear achromatic is subdivided into individual surfaceportions whose dimensions are below the resolution limit of the humaneye. Within a surface portion the local grating period (line spacing)varies in accordance with a predefined, for example linear function,over a given period range. It is further known in regard to anachromatic hologram for the grating period to be locally stochasticallyaltered, see for example the book "Optical Holography", edited by P.Harriharan, Cambridge Studies in Modern Optics, pages 144 ff, ISBN 0 52131162 2.

All those methods suffer from the common disadvantage that, although anachromatic impression can admittedly be produced for a predeterminedviewing angle, pronounced colour fringes appear in the adjoining viewingangles. If moreover the viewing range over which a representation is toappear achromatic is increased by a large period extent, the brightnesswhich can be perceived by an observer decreases noticeably as theincident light is distributed over a correspondingly larger angularrange.

SUMMARY OF THE INVENTION

According to the present invention there is provided an opticallyvariable surface pattern as set forth in claim 1.

Embodiments of the present invention provide an optically variablesurface pattern which is difficult to forge, with at least onerepresentation of a graphic configuration, wherein the representationproduces an achromatic impression when viewed in visible light over acertain angular range without noticeable colour fringes occurring in theadjoining angular ranges.

For the purposes of describing the general idea of the invention, let itbe established as the initial situation that a surface pattern containsat least n=2 representations. The surface pattern is thereforesubdivided into first and second surface portions. The first surfaceportions serve to produce the first representation and the secondsurface portions serve to produce the second representation. Bothrepresentations are to be achromatic, that is to say they are to bevisible in the colour of the light illuminating them and they are alsonot to produce changing colour effects when the surface pattern isturned or tilted. In accordance with geometrically optical notions, thespecified object is attained in that the surface portions belonging tothe first representation are in the form of reflecting surfaces whichare inclined through a first predetermined angle of inclination α₁ witha first predetermined azimuthal orientation Φ₁ with respect to the planeof the surface pattern, or they are in the form of diffusely scatteringmatt structures. Instead of a diffusely scattering matt structure, it isalso possible to provide a mirror surface which is disposed in the planeof the surface pattern. The reflecting surfaces belonging to the secondrepresentation are inclined relative to the plane of the surface patternin another azimuthal orientation Φ₂ through a second angle ofinclination α₂. With the predetermined viewing direction an inclinedsurface portion produces a light pixel whereas a matt structure ormirror surface produces a dark pixel. With an angle of inclination of15° and an extent of the surface portions of a maximum of 100micrometres however there are differences in respect of height relativeto the plane of the surface pattern of about 27 micrometres. Thereforeeach inclined surface portion is broken down into an organisation ofnarrower surface portions which are arranged in parallel side-by-siderelationship, with the same angle of inclination α₁ and α₂ respectively.This organisation which replaces the original surface portion is arelief structure and in cross-section is of a sawtooth-shaped profilewhose grating period and profile height are matched to each other insuch a way that the light diffracted at the sawtooth-shaped profile ofthe relief structure behaves in a first approximation similarly to thelight reflected at the original inclined surface portion. Such abehaviour is achieved if the profile height of the sawtooth isapproximately an integral multiple of half the optical path length ofthe light, in which respect that condition is possibly to be adapted tothe angle of incidence of the light. That condition is approximatelysimultaneously met for an optical path length of 3.3 or 7.15 micrometresfor example for the three wavelengths in the visible range λ₁ =450 nm,λ₂ =550 nm and λ₃ =650 nm. If the reflecting surface is covered with alacquer layer with an optical refractive index of 1.5, that gives aprofile height which is reduced by the factor n=1.5, of 1.1 and 2.37micrometres respectively.

In the case of a surface pattern embodying the invention each of the tworepresentations is visible from only one viewing direction, in whichcase the two representations do not interfere with each other.

BRIEF DESCRIPTION OF THE FIGURES

FIG. 1 shows, as a function of the diffraction angle θ, the standardizedintensities I of the diffraction orders of a conventional grating withsinusoidal profile shape, wherein the light is incident perpendicularly;

FIG. 2a shows the standardized intensities of the diffraction orders fora grating embodying the invention with a sawtooth-shaped profile shape;

FIG. 2b shows the standardized intensities of the diffraction orders foranother grating embodying the invention with a sawtooth-shaped profileshape;

FIG. 3 shows a surface pattern;

FIG. 4 shows three representations of graphic configuration;

FIG. 5 shows the surface pattern in the form of a composite laminatewith surface portions having a grating structure of a sawtooth-shapedprofile shape;

FIG. 6 shows details of a further surface pattern;

FIG. 7 shows a further surface pattern; and

FIG. 8 shows a surface pattern made up of lines.

DETAILED DESCRIPTION OF THE INVENTION

From an optical-diffraction point of view embodiments of the inventionafford the teaching of using grating structures with a large gratingperiod, that is to say a small number of lines, so that many diffractionorders can occur in the visible range, to produce an achromatic opticalimpression in respect of the two representations. In addition theprofile shape is to be such that the maximum possible proportion of thediffracted light is diffracted into higher diffraction orders. So thatthe ratio between the light which is diffracted into positivediffraction orders and the light which is diffracted into negativediffraction orders is as high as possible, grating structures with anasymmetrical profile shape and in particular a sawtooth profile shapeare to be used. These ideas are described in greater detail hereinafter.

In the case of grating structures with a small number of lines manydiffraction orders can exist in accordance with equation (1). With anumber of lines of 100 lines/mm and with a wavelength of λ=550 nm, withperpendicular incidence the diffraction orders m=-18, -17, -16, . . .-1, 0, +1, . . . , +17, +18 can occur, that is to say 37 diffractionorders within the full diffraction angle range of -90° to +90°. Theangle spacing between adjacent diffraction orders is typically 3-4°.

The diffraction angles θ_(m) are determined in accordance with equation(1) by the period d of the grating structure. The levels of intensity ofthe light which is diffracted into the various discrete diffractionorders are determined by the profile shape and the profile height of thegrating structure. By suitable selection of those two parameters, it ispossible to control the distribution of intensity of the diffractedlight in such a way that light of the wavelength λ is diffracted for themajor part into diffraction orders whose diffraction angles θ_(m) areclose together in a narrow angle range ψ. The incident polychromaticlight is also diffracted into the narrow angle range ψ for all differentwavelengths λ. The grating structure therefore appears to the viewerwithin the angle range ψ light and achromatic, in the colour of thelight illuminating the grating structure, while it is dark outside theangle range ψ.

FIG. 1 shows as a function of the diffraction angle θ the standardisedintensities I of the diffraction orders of a conventional grating with asinusoidal profile shape, wherein the light is incident perpendicularly.The grating has a number of lines of 1000 lines/mm and a profile heightof 155 nm. The spectra are calculated for the three wavelengths λ₁ =450nm, λ₂ =550 nm and λ₃ =650 nm, corresponding to the colours blue, greenand red. The light of the three colours is diffracted into discreteangles θ_(m) which are far apart. There are two positive diffractionorders for the blue light, while there is only one for the green and thered light. As the grating has a sinusoidal and thus symmetrical profileshape, the same amount of light is also diffracted into negativediffraction angles θ_(-m). When the grating is turned and/or tilted, aviewer sees the surface occupied by the grating in changing colours.

FIGS. 2a and 2b show the standardised intensities of the diffractionorders for two gratings embodying the invention with a sawtooth-shapedprofile shape. The gratings both have a number of lines of 150 lines/mmbut different profile heights h of 1.8 μm and 1.3 μm respectively. It isclearly apparent that the light of all three colours is diffracted witha high level of intensity into a narrow angle range ψ at about +32° and+26° respectively. In the first case the angle range ψ coversapproximately angles θ of 30°-35°. Only very little light is diffractedinto the other, both positive and negative, diffraction orders.Practically no light is also diffracted into the angle range at -32° and-26° respectively as, because of the asymmetrical profile shape of thecorresponding grating, it is readily possible to achieve a ratio of thelight which is diffracted into the positive diffraction orders to thelight which is diffracted into the negative diffraction orders, of atleast 100:1. Therefore, each of those two gratings appears to a viewerin a relatively narrow angle range ψ as an achromatic surface while inthe remaining angle ranges it appears as a dark surface, withoutnoticeable colour fringes occurring when the grating is turned and/ortilted. If the gratings are covered with a lacquer layer with arefractive index of n=1.5 the profile height h can be reduced by afactor n to 1.2 μm and 0.89 μm respectively. By virtue of the selectedprofile shape and profile height of the gratings the light is diffractedinto high positive diffraction orders with a high level of efficiency,more specifically green light approximately into the plus tenth.

The angle range ψ in which the viewer perceives the grating structuresas being achromatic is determined by the number of lines: the smallerthe number of lines, the narrower is the achromatic angle range ψ. Thediffraction angle θ_(m) with the highest level of intensity increaseswith the profile height or the angle of inclination of the sawtooth,with a predetermined number of lines, as can be seen from FIGS. 2a and2b.

As is to be noted from FIGS. 2a and 2b, discrete diffraction ordersstill occur, but only a few diffraction lines which are associated withthe various spectral colours involve noticeable intensity within theangle range ψ, under normal illumination. Those diffraction lines arenow so close together in terms of angle that the surface portionoccupied by grating structures of that kind, when illuminated with whitelight and viewed from any direction within the angle range ψ, does notappear in changing colours but appears to the viewer as always remaininglit white or in other words as an achromatic surface.

The concentration of the diffracted light into a closely defined anglerange ψ causes the illuminated surface portion to flash brightly whenthe observer tilts or turns the surface pattern. That effect cannot beachieved with other known optical-diffraction surface reliefs as therethe light is diffracted in spectrally resolved form into a relativelylarge angle range. In addition the grating with such a large profileheight cannot be copied with a holographic contact copy to produce asurface relief as with the holographic contact copy the profile heightof the relief, for example resulting in photoresist, would typically beonly about 0.1 to 0.2 μm. In addition other forms of the holographiccopy procedure for producing a surface relief (see for example thedescription of the contact copy process and the two-step process in S.P. McGrew, Hologram Counterfeiting: Problems and Solutions, SPIE vol.1210 Optical Security and Anticounterfeiting Systems 1990) also involvelosing the pronounced asymmetry of the grating structure, which is alsohighly important so that the light is diffracted into high diffractionorders with a high level of efficiency. In addition a given profileshape is also a prerequisite for achieving the achromatic effect.

Embodiments of the invention are now described in greater detailhereinafter with reference to the drawings.

FIG. 3 shows a surface pattern 1 which is subdivided matrix-like inton*m areas or fields 2. Each area 2 is subdivided into k=3 surfaceportions 3, 4 and 5. The totality of the surface portions 3, 4 and 5respectively of all areas 2 contains a respective one of k=3representations 6, 7 and 8 (FIG. 4). The azimuth angle Φ denotesrelative to a reference direction 9 an orientation direction 10 withinthe plane of the surface pattern 1. The direction 11 denotes thedirection of incidence of light which is incident on the surface pattern1, a cone 12 denotes the angle range ψ into which light diffracted atthe surface portions 3 of the representation 6 is predominantlyfocussed.

FIG. 4 shows the three representations 6, 7 and 8 which represent thegraphics "Schweiz", "Suisse" and "Svizzera". The graphics are light on adark background. The representations 6, 7 and 8 are also subdividedmatrix-like into small n*m grid areas which are either light or dark. Asurface portion 3 (FIG. 3) is associated with each grid area of therepresentation 6, a surface portion 4 is associated with each grid areaof the representation 7, and so forth.

If the grid area of the representations 6 is dark, the associatedsurface portion 3 contains a matt structure which diffusely scatters theincident light, or a flat, non-inclined mirror surface so that itappears dark for all angles or for all angles with the exception of thereflection angle. If the grid area is light, the associated surfaceportion 3 contains a grating structure 13 (FIG. 5) which diffracts thelight incident in the predetermined direction of incidence 11 (FIG. 3),predominantly into the angle range ψ represented by the cone 12. Theorientation and the spread angle ψ of the cone 12 are defined by theazimuth angle Φ₁ of the grating structure 13 or the profile shape andthe profile height of the grating structure 13. The grating structure 13of the surface portions 3 has a comparatively small number of lines oftypically 100 to 250 lines per millimetre and an asymmetrical profileshape, preferably a sawtooth profile shape, as is shown in FIG. 5. Byvirtue of the small number of lines, typically at least ten diffractionorders occur for visible light. The profile shape is now predeterminedin such a way that the light in the visible range is diffracted with ahigh level of diffraction efficiency into as few as possible but highdiffraction orders. Admittedly under some circumstances light is alsosomewhat diffracted into the other diffraction orders. The intensitythereof is very low so that it is not noticeable to a viewer. As thelight is diffracted for the major part into diffraction angles θ_(m) ofhigher order m and as the diffractions angles θ_(m) for differentwavelengths, for example λ₁ =450 nm, λ₂ =550 nm and λ₃ =650 nm overlap,the achromatic behaviour on the part of the grating structure 13 isachieved in the predetermined angle range ψ: in the angle range ψ therepresentation 6 appears light while outside the angle range ψ therepresentation 6 is not visible. In addition, no observable changingcolour effects as are typical in relation to optical-diffractionstructures occur when the surface pattern 1 is turned and/or tilted. Theterm turn is used to mean that the surface pattern is turned about anaxis which is perpendicular to the plane of the surface pattern. Theterm tilt is used to mean that the surface pattern is turned about anaxis which is disposed in the plane thereof. To sum up it is found thatthe representation 6 can only be viewed from the predetermined solidangle range ψ with a fixed direction of incidence 11 of the light. Inthat case the representation 6 appears in the form of an imageconsisting of light and dark points which generally involve the colourof the reflection layer 15 (FIG. 5) used to cover the grating structures13 and/or the cover layer 16 (FIG. 5).

The representation 7 is embodied with a similar grating structure 13 tothat of the representation 6. The azimuth angle ψ thereof howeverinvolves an angle difference of preferably 180° relative to the azimuthangle Φ₁, of the representation 6 so that the representation 7 isvisible from a different solid angle range ψ, in which case it can alsobe perceived as an image composed of light and dark, achromatic points.It is possible to conceive of different image contents for therepresentations 6 and 7 from those adopted in FIG. 4, in which the angledifference of 180° provides advantageous effects. The prerequisite fornonetheless only a respective one of the two representations 6, 7 beingperceptible is a high degree of asymmetry of the relationship of thelight which is diffracted into positive diffraction orders and the lightwhich is diffracted into negative diffraction orders. That ratio istypically at least 100:1 with a profile shape for the grating structure13, which is optimised in relation to asymmetry.

The representation 8 is made with a grating structure 13 which has ahigher number of lines of typically 800 and more lines per millimetre.By virtue of that high number of lines the representation 8 haspronounced optical-diffraction effects, that is to say changing colourswith a high level of luminosity when the surface pattern 1 is turnedand/or tilted.

It is not entirely impossible for the representations 6 and 7 to exhibitslight colour fringes in the transition from the visible angle range ψof the cone 12 into the invisible angle range. There is however thecentral region of the cone 12 in which the image impression ispronouncedly achromatic. In the case of the representation 8 in contrastthere is no achromatic range, but that representation 8 appears in acolour which is well-defined from the optical-diffraction point of view,in any viewing angle.

As shown in FIG. 5 in cross-section, the surface pattern 1 isadvantageously in the form of a composite laminate. The compositelaminate is formed by a first lacquer layer 14, a reflection layer 15and a second lacquer layer, the cover layer 16. The totality of thegrating structures 13 and the matt structures of the surface portions3-5 are embodied in the form of microscopically fine relief structures.The lacquer layer 14 is advantageously an adhesive layer so that thecomposite laminate can be glued directly onto a substrate. The coverlayer 16 advantageously completely levels off the relief structures. Inaddition in the visible range it preferably has an optical refractiveindex of at least 1.5 so that the geometrical profile height h gives anoptically effective profile height which is as large as possible. Thecover layer 16 also serves as a scratch-resistant protective layer.

The subdivision of the representations 6 (FIG. 4), 7, etc. into gridareas does not have to be regular. That depends on the motifs of therepresentations 6, 7 etc. The surface portions 3 (FIG. 3), 4, etc. mayalso locally vary in shape and size. In order for example to increase alocally higher degree of brightness of a predetermined grid area of therepresentation 6, the surface portion 3 associated with the grid area ofthat representation may be increased within certain limits at theexpense of the adjacent surface portions 4 or 5 of the otherrepresentations 7 or 8.

The subdivision of the representations 6, 7 and so forth into grid areaswith light and dark pixels is not always meaningful or necessary. Eachrepresentation 6, 7 and so forth includes light and dark image regions.In embodiments of the invention, associated with the light image regionsare surface portions 3, 4 and so forth with a grating structure 13 (FIG.5) with predetermined grating parameters. The surface of therepresentations 6, 7 and so forth, which is occupied by the dark imageregions, is provided on the surface pattern 1 (FIG. 3) either in theform of a surface portion with a matt structure or in the form of areflecting non-inclined surface portion or is associated as a surfaceportion 3, 4 and so forth with a grating structure 13 with other gratingparameters, with a light image region of another representation 6, 7 andso forth. Three further embodiments for achieving particular opticaleffects will now be described hereinafter, in which the surface portion3, 4 and so forth associated with a dark image region of therepresentations 6, 7 and so forth also includes a diffracting reliefstructure.

FIG. 6 shows two surface portions 3a and 3b of the surface pattern 1,wherein the surface portions 3a are associated with light image regionsof the representation 6 (FIG. 4) while the surface portions 3b areassociated with dark image regions thereof. The surface portion 3acontains a microscopically fine relief structure which diffractsperpendicularly incident light 17 into a first direction 18 in space,which is defined by the pair of angles (Φ₁, θ₁). The surface portion 3bcontains a microscopically fine relief structure which diffractsperpendicularly incident light into a second direction 19 in space whichis defined by the pair of angles (Φ₂, θ₂). The absolute differencebetween the two azimuth angles |Φ₁ -Φ₂ | is typically at least 45°. Thatprovides that, when light is incident perpendicularly, the surfaceportion 3a appears light and the surface portion 3b appears dark to aviewer looking onto the surface pattern 1 from the direction 18 inspace. In contrast the surface portion 3a appears dark and the surfaceportion 3b appears light to a viewer looking onto the surface pattern 1from the direction 19 in space. The representation 6 is thus perceptiblewith reversed contrast from the two directions 18 and 19 respectively.Each surface portion 3a, 3b and 4 has a largest linear dimension of atmost 0.3 mm so that it is perceptible by eye at most as a structure-lesspoint.

In a further embodiment for example the second representation 7 (FIG. 4)comprises two different motifs which are disposed in side-by-siderelationship and do not overlap. The two motifs are to be visible fromdifferent viewing directions. In that case it is possible for bothmotifs to be disposed in the surface portions 4 which are associatedwith the grid areas of the second representation. The parameters of therelief structures of the first motif and those of the second motif arethen different and can be established independently of each other. Thesame solution can also be used in relation to more than two motifs whichdo not overlap.

In addition for example the surface portion 4 associated with a darkgrid area of the second representation 7 (FIG. 4) may contain the samerelief structure as the adjacent surface portion 3 (FIG. 3) which isassociated with a light grid area of the first representation 6. Thatmakes it possible to increase the brightness of the corresponding gridarea of the representation 6. That way of enhancing brightness ispossible within the limits defined by the graphic contours of therepresentations 6, 7.

FIG. 7 shows the surface pattern 1 which as an example of the graphicconfiguration has a large rectangle, a triangle, a circular area and asmall square. The triangle, the circular area and the square arearranged within the large rectangle without overlapping. The largerectangle corresponds to the first representation 6 (FIG. 4), thetriangle corresponds to the second representation 7, the circular areacorresponds to the third representation 8 and the square corresponds toa fourth representation. Those surface parts of the large rectanglewhich are not covered by the triangle, the circular area or the squarerepresent a single surface portion 3 or are subdivided into surfaceportions 3 and 20. The area occupied by the triangle contains surfaceportions 3, 4 and 20. The circular area contains surface portions 3, 5and 20. The area occupied by the square represents a single surfaceportion 21. The surface portions 3 contain a grating with a number oflines of 1000 lines/mm and a symmetrical profile shape so that the largerectangle exhibits rainbow colour effects when the surface pattern 1 isturned and/or tilted. The surface portions 4 contain a grating with anumber of lines of 250 lines/mm whose azimuth angle is Φ₁ (FIG. 6) andwhich has an asymmetrical profile shape whose profile height is sopredetermined that the triangle appears achromatically light to a viewerlooking from the predetermined direction 18 in space (FIG. 6). In otherdirections in space, the triangle is scarcely visible as the surfaceportions 3 appear substantially lighter than the surface portions 4. Thesurface portions 20 contain a matt structure or a mirror surface whichis flat relative to the plane of the surface pattern 1. The surfaceportions 5 contain the same grating as the surface portions 4, but withanother orientation in respect of the azimuth angle Φ₂ (FIG. 6) . Thecircular area thus appears achromatically light from another direction19 in space (FIG. 6). The surface portion 21 of the square also containsa relief structure which appears achromatically light from anotherpredetermined direction in space. The relationship of the areaproportions of the surface portions 3, 4, 5 and 20 determines therelative brightness of the four different representations. The greatestbrightness is exhibited by the square whose full area is provided with arelief structure with an asymmetrical profile shape, which has a highlevel of diffraction efficiency. The levels of brightness of thetriangle and the circular area, as well as the large rectangle,essentially depend on the proportional size of the area occupied by thesurface portions 20. The relative brightnesses thereof can thus becontrolled by means of using surface portions 20. With the exception ofthe area occupied by the square the individual surface portions 3, 4, 5and 20 are of linear dimensions of at most 0.3 mm so that they are notindividually perceptible by eye from a normal viewing distance of 30 cm.In the illustrated example they are shown on an enlarged scale forreasons relating to clarity of the drawing. The pronounced achromaticeffect, the asymmetry of the diffraction effects and relative brightnesslevels serve as different security features.

Relief structures which produce an achromatic effect can also be usedfor a surface pattern 1 in which subdivision of the representations intogrid areas is not necessary or is not meaningful. FIG. 8 shows thesurface pattern 1 with a star comprising at least two narrow lines 22,23 which do not cross each other. The lines 22, 23 belong to twodifferent representations,that is to say the line 22 is to be visiblefrom a different viewing direction from the line 23. The line 22 has afirst relief structure and the line 23 has a second relief structure toproduce an achromatic effect, wherein the parameters of the two reliefstructures are selected to be different so that the lines 22 and 23 arevisible from different directions in space. When the surface pattern isturned and/or tilted the star therefore exhibits a 15 kinematic effectinsofar as the brightness levels of the lines 22 and 23 change. Thekinematic effect can be refined with an increasing number of lines 22,23.

Stated in generalised terms the surface pattern 1 can be subdivided intosurface portions 3 (FIG. 3), 4, 5 and so forth of any shape which do nothave to be either continuous or mutually adjoining, wherein groups ofsurface portions 3, 4, 5 and so forth which have the same reliefstructure are associated with predetermined representations 6 (FIG. 4),7, 8 and so forth. In that way it is possible to integrate into thesurface pattern 1 in particular representations which, similarly toconventional engraving, are made up of a plurality of lines. If lines ofdifferent representations overlap that nonetheless does not givetroublesome optical effects as the area occupied by the points ofintersection is very small in terms of proportion. The area of thesurface pattern 1, which remains between the lines of the variousrepresentations, can be in the form of a matt or a reflecting surface.

The surface pattern 1 which has representations consisting of lines canbe produced in a technologically simple manner in accordance with theteachings of European patent specification EP 330 738 or Swiss patentspecification CH 664 030.

It will be appreciated that it is possible for the chromaticrepresentations to have superimposed thereon motifs which in terms ofproportion advantageously occupy only a very small area such as forexample guilloche patterns or microscripts which exhibit kinematiccolour effects. Such kinematic optical effects are known from Europeanpatent documents EP 105 099, EP 375 833 or EP 490 923 and products whichare marketed under the name KINEGRAM®. If the representation 6 (FIG. 4)contains a first motif with a grating structure which achromaticallydiffracts impinging light into the predetermined angle range ψ and asecond motif with a grating structure which for example diffracts thegreen spectral component of the impinging light into a direction whichis within the angle range ψ, then the two motifs reference each other ina manner which is easily recognisable for an observer. It is clear fromFIGS. 1 and 2a that such referencing is possible for example with asawtooth-shaped grating with a number of lines of 150 lines/mm and aprofile height of 1.2 μm and a sinusoidal grating with 1000 lines/mm anda profile height of 0.155/1.5=0.1 μm if the gratings are covered withthe lacquer layer 16 (FIG. 5) with a refractive index n=1.5. The twograting structures are arranged in the surface portions 3 (FIG. 3) whichbelong to the representation 6. In the case of holographic copyingprocesses at least the diffraction angles θ of the two gratingstructures change in different ways so that the effect of thereferencing is lost.

What is claimed is:
 1. An optically variable surface pattern,comprising:juxtaposed areas subdivided into reflective surface portions,each of the reflective surface portions comprising one of a diffractinggrating structure, a matte structure and a non-inclined flat mirrorsurface; a laminate in which said reflective surface portions areembedded; representations of graphic configuration which include atleast light and dark image regions, the reflective surface portionscorresponding to the light image regions being comprised of thediffracting grating structure associated with a particular one of therepresentations such that the representations are visible at differentviewing directions upon being illuminated by visible light; and thelight image regions of at least one representation comprised of thereflective surface portions having a first grating structure with a linenumber of between about 100 and 250 lines per millimeter and with aprofile height such that upon being illuminated, the light image regionsof said at least one representation appear achromatically bright withinat least one first cone with a predetermined first solid angle andappear achromatically dim outside of the first cone.
 2. A surfacepattern according to claim 1, wherein the dark image regions of saidrepresentation is composed of the surface portions having a secondgrating structure differing in at least one parameter from the firstgrating structure of the light image surface portions.
 3. A surfacepattern according to claim 2, wherein the second grating structure issinusoidal and has a line number of at least 800 lines per millimeter.4. A surface pattern according to claim 2, wherein:the first and secondgrating structures differ in the optical profile height, which is theproduct of the geometrical profile height and the index of refraction ofa cover layer of the laminate which covers the first and second gratingstuctures; and the difference in optical profile height is at least 0.5micrometer.
 5. A surface pattern according to claim 1, wherein the darkimage regions of said representation is comprised of the surfaceportions having a second grating structure with a line number betweenabout 100 and 250 lines per millimeter and with such a profile shape andsuch a profile height that upon being illuminated the light imageregions of said representation appear achromatically bright in at leastone second cone with a predetermined second solid angle range andachromatically dim outside of the second cone, and said first and secondcones do not overlap, so that said representation is visible in reversedcontrast from two predetermined viewing directions.
 6. A surface patternaccording to claim 5, wherein for said representation, each of saidfirst and second asymmetric grating structures has a sawtooth-shapedprofile.
 7. A surface pattern according to claim 1, wherein for saidrepresentation, said first grating structure has a sawtooth-shapedprofile.
 8. A surface pattern according to claim 1, wherein the darkimage regions of said representation is comprised of the surfaceportions having the matte structure.
 9. A surface pattern according toclaim 1, wherein the dark image regions of said representation iscomprised of the surface portions having the non-inclined flat mirrorstructure.
 10. A surface pattern according claim 1, wherein the surfaceportions of the areas associated with the light image regions arearranged along lines and the surface portions of the areas associatedwith the dark image regions are arranged between the lines.
 11. Anoptically variable surface pattern, comprising:juxtaposed areassubdivided into reflective surface portions, each of the reflectivesurface portions comprising one of a diffracting grating structure, amatte structure and a non-inclined flat mirror surface; a laminate inwhich said reflective surface portions are embedded; at least first andsecond representations of graphic configuration which include light anddark image regions; first surface portions of areas corresponding to thelight image regions of the first representation having a structure of afirst kind and the first surface portions of areas corresponding to thedark image regions of the first representation having a structure of asecond kind; second surface portions of areas corresponding to the lightimage regions of the second representation having a structure of a thirdkind and the second surface portions of areas corresponding to the darkimage regions of the second representation having a structure of afourth kind; the structures of the first and third kind being gratingswith a line number between about 100 and 250 lines per millimeter andwith a profile shape and profile height such that upon beingilluminated, the light image regions of said first and secondrepresentations appear achromatically bright within at least one firstand one second cone, with a respective first and second solid anglerange and appear achromatically dim outside of the first and secondcone, and said first and second cones do not overlap.
 12. A surfacepattern according to claim 11, wherein the structure of the second kindis said matte structure.
 13. A surface pattern according to claim 12,wherein the structure of the fourth kind is said matte structure.
 14. Asurface pattern according to claim 11, wherein the structure of thesecond kind is said non-inclined flat mirror structure.
 15. A surfacepattern according to claim 14, wherein the structure of the fourth kindis said non-inclined flat mirror structure.
 16. A surface patternaccording to claim 11, wherein the structure of the fourth kind is saidmatte structure.
 17. A surface pattern according to claim 11, whereinthe structure of the fourth kind is said non-inclined flat mirrorstructure.
 18. A surface pattern according claim 11, wherein the surfaceportions of the areas associated with the light image regions arearranged along lines and the surface portions of the areas associatedwith the dark image regions are arranged between the lines.